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Trane CGAM chiller problems

I have a Trane CGAM chiller (CGAM070F), serial # U09M13536, Refrigerant R-410A, that has one circuit (two parallel compressors) with the capacity limited by compressor discharge temp. I observed that when the 2nd compressor in the affected circuit came on, the high-side pressure built up to 417.4 psig and discharge temp rose to 120F and then the circuit drops to only one compressor and the display board (Tracer CH530) again indicates capacity is again limited by high discharge temperature. Checking the manual, is simply indicates the discharge temperature should be 63F or more and the condensing pressure should be 420 to 440 psig. However, it never gets to that condensing pressure range when the capacity again is limited.

The other circuit of the chiller indicates it is off on high pressure and will not reset. This chiller service call came after the 2nd hit and severe rain downpour from the remnants of Isaac.

I have the same model/size chiller at another location and experiences the same capacity limit by high discharge temperature on one circuit and had the condenser cleaned with no change. Both chillers have all condenser fans operations and showing 100% air flow.

Yes, instead of simply checking via putting my hand over the top, I got a ladder and took a look. All 3 fans appear to all be running the same speed and I verified that all are pushing strong air upwards (no free spinning blades). Was told by one of my fellow techs who worked on the unit before that the first fan is variable speed, but it appears to be running as fast as the other two.

Is the 120F discharge what you are reading with a therm. or the 530 display? The high press. switch has 2 sets of contacts, 1 norm open and 1 norm closed. The norm closed breaks power to the contactor and the norm open sends a signal to the 530 that the high press switch is tripped, been a few problems with the latter switch. Looks can be deceiving with rotation of the fan, sometimes, it is better to stick your hand above to feel the air blowing out. Those dead fans can be spinning just as fast as the others, only in the wrong direction.

I did place my hand over all 3 fans while on the ladder, all blowing hard upwards. First technician to the chiller reset the high-pressure switch and the compressors came on. Now the "reset diagnostics" doesn't clear the high pressure fault message and the circuit is off.

Another technician checked the high pressure switch and disconnected one set of contacts. That circuit came on and the chilled water supply is now 45F. The other circuit still has running limit (limited by high discharge temperature). Luckily, there is enough extra capacity to handle the load.

These chillers use a special fast responding suction temperature sensor for controlling superheat. You have one of the earlier CGAM chillers and they had a problem with these sensors. As Jay said, check the accuracy of your suction temp sensor along with all other sensors. With the circuit off and cooled down, it should read about the same as the ambient temperature sensor.